Thermionic energy converters are used to convert thermal energy at temperatures between 1200 K. and 2500 K. to electric energy without mechanical movable parts. A thermionic converter works as a heat machine between above stated source temperature and a drain temperature of typically 800 K. The converter consists of two electrodes of metal or other appropriate conducting material, one of them at the source temperature, the emitter, and the other at the drain temperature, the collector. The electrodes are located near each other in a vacuum or at low pressure, and the emitter emits a current of electrons to the collector, by it being held at a higher temperature through supply of thermal energy from the outside, for example from a flame or other heat source. The electrodes frequently constitute a part of the external vacuum tight wall or shroud of the converter, and are separated by insulating material. In thermionic converters cesium vapor is normally used with a pressure of magnitude of 1 mbar to increase the electron emission from the emitter and to reduce the problems with space charge in the converter, so that larger current densities can be obtained from the converter. The emission from the emitter is increased by cesium lowering the work function for the electrons from the surface. In the same manner the work function is decreased on the collector, which has very great importance for the function of the converter. Detailed descriptions of thermionic converters are found in the references: G. N. Hatsopoulos and E. P. Gyftopoulos, Thermionic Energy Conversion, Vol. I (MIT Press, Cambridge, Mass., 1973) as well as G. N. Hatsopoulos and E. P. Gyftopoulos, Thermionic Energy Conversion, Vol. II (MIT Press, Cambridge, Mass. 1979).
When the converter gives an output the work function of the collectors corresponds to a loss, i.e. the electrons from the emitter lose the corresponding energy in the form of heat in the collector. The factor of merit for thermionic converters, the so called barrier index, is composed of the work function of the collector and the so called arc voltage drop in the converter. The barrier index is positive and must be as small as possible. These two parts in the barrier index represent the main losses in the converter during normal operation. The work function of the collector normally gives the largest contribution to the barrier index, and a low work function of the collector is consequently of extremely great importance for the manufacture of efficient thermionic converters. Frequently simple metals are used with work functions of 4-5 eV as collector material, for example molybdenum. In operation such a collector is covered with a thin layer of cesium metal (smaller than a simple layer of atoms, a so called monolayer) or of cesiumoxid. This layer lowers the work function of the collector to 1.6-1.8 eV in normal operation.
It is further known for example through the U.S. Pat. No. 4,747,998, that it is possible to keep an alkali metal such as cesium in a graphite container to obtain a regulated pressure of alkali metal in a thermionic converter.
The purpose of the invention and its most important features
The purpose of the present invention is to achieve a thermionic energy converter of the type mentioned in the introduction, which exhibits a very low work function of the collector which entails a more effective energy conversion in the thermionic converter. This has been achieved by the collector at least partly being covered by a thin layer of a material, for example carbon, which is able to interact with said thermionic material and form electronic excited states of this, and that by operation a layer of excited thermionic material is maintained on the collector surface.